You're right to be concerned.
The Fair Play for Women site has explored the numbers and offending behaviour of trans prisoners. Through Freedom of Information requests they established that last year 60 out of a total of 125 trans prisoners (all of whom were male) were sex offenders.
This is a very much larger proportion than in the general prison population. It suggests that either transwomen are more likely to be sex offenders or that sex offenders are identifying as transwomen for reasons of their own.
Or possibly a bit of both. The FBI has said that fetishistic cross dressing is the most common paraphilia found in sex offenders, so there's that. But there are many potential benefits for a sex offender of pretending to identify as a woman. Top of them is surely to gain access to women by being moved to a women's prison. But there are others.
Here's what the British Psychological Society said about the issue in the prison estate:
... psychologists working with forensic patients are aware of a number of cases where men convicted of sex crimes have falsely claimed to be transgender females for a number of reasons:
- As a means of demonstrating reduced risk and so gaining parole;
- As a means of explaining their sex offending aside from sexual gratification (e.g. wanting to ‘examine’ young females);
- Or as a means of separating their sex offending self (male) from their future self (female).
- In rare cases it has been thought that the person is seeking better access to females and young children through presenting in an apparently female way.
Such strategies in no way affect risk and indeed may increase it.
Some people falsely believe that taking oestrogen and blocking androgen in males will reduce risk of offending, however this is not necessarily the case.
Consequently the Society recommends that the Government give appropriate assistance to transgender people within the criminal justice system; while being extremely cautious of setting law and policy such that some of the most dangerous people in society have greater latitude to offend.
On a more practical note, transgender prisoners get more privacy in the showers and maybe a cell of their own. So there are plenty of reasons a prisoner might pretend.
This is one of the concerns women have about Self ID. Even if every man who identifies as a woman were a saint, allowing one group of men into woman only spaces essentially lets them all in. Because predators exploit loopholes.